In a bittersweet commencement ceremony that highlighted how closely its fate is intertwined with the city around it, Boston University bestowed degrees on nearly 6,700 graduates, including posthumous degrees awarded to Lu Lingzi, the graduate student killed in the Marathon bombings, and Binland Lee, a senior who died in a fire in her off-campus apartment last month.

As clouds moved over Nickerson Field, Robert A. Brown, Boston University’s president, opened the ceremony by acknowledging that “we celebrate a scant four weeks and six days” after the Marathon bombings. He also honored Zhou Danling, Lu’s friend and a fellow BU graduate student seriously hurt in the bombings. Brown added that “we lost a prospective student,” 8-year-old victim Martin Richard.

When provost Jean Morrison announced the posthumous degrees for Lu and Lee and their photographs appeared on giant video screens, the sea of graduates and their families rose in a long standing ovation.

Despite those somber notes, and a streak of tragedies that took the lives of 11 BU students since last spring, Sunday’s graduation was largely joyous, with newly minted graduates cheering wildly for themselves and waving madly at cameras.

Many had decorated their mortarboards to celebrate their majors, their fraternities or sororities, or their artistic visions. Multiple students wore hand-painted images of the city skyline — Kenmore Square’s Citgo sign prominently highlighted — while others declared sentiments from “hire me,” to “thank you mom and dad,” to, “Boston strong, BU smart.”

The commencement speaker was Wendy Kopp, whose senior thesis in college led her to found Teach for America, a program that brings elite college graduates to teach in low-income schools. She told students that they were about to become stewards of the BU legacy of promoting social justice, represented most notably by Martin Luther King Jr., who received his doctorate in theology from the university.

Because “there’s no how-to guide for how to change the world,” Kopp set out to debunk several myths about how to have an impact. Among them, she said, is the misconception “that having an impact is about being first,” whether by starting an organization or inventing a technology.

“Our world needs more copycats,” she said, describing an educator who studied some of the best schools in other countries before building a wonderful school in a Mumbai slum, where most children’s parents are illiterate.

She said that former president Bill Clinton was proudest as governor of Arkansas when his state was the second to do something. “Because that meant an idea could be replicated on a scale that would make a real difference in people’s lives,” she said.

Kopp also harkened back to the Marathon bombings, reminding the graduates of what Lu’s roommate, Jing Li, proclaimed at her memorial service: “We will keep running to finish the race for you and we will try to realize your unfinished dream.”

Kendrea Collins, an advertising major from Memphis, and Stephanie Derrick, a sociology major from Atlanta, described how after the recent senior breakfast, many students brought flowers to the MLK memorial at the center of campus in honor of their classmate Lee.

Collins is headed to an internship in Australia while Derrick is planning to apply to graduate programs.

“I am rejuvenated, knowing that I made it here,” Derrick said. “I’m a first generation college student. I’m excited about life, to be honest . . . I’m excited about growth and what comes with it.”

Collins added, “this is probably really cliché, but watch out for the class of 2013. We’re just beginning and we’re going to change the world.”

Actor Morgan Freeman, an honorary degree recipient, sparked delighted shrieks from the graduates and chants demanding, “Speech! Speech!” So he took the podium briefly to tell them, “I have nothing new to say to you,” because “you already know that dedication, hard work, and perseverance get positive results. So I can only say to you, congratulations!”

Mayor Thomas M. Menino received a standing ovation as he accepted an award for his years of leadership. Brown praised Menino for his commitment to education, particularly, he said, for recognizing “the role universities and colleges play in strengthening the cultural life of the city.”

Brown announced that the university’s scholarship for Boston public school graduates will be renamed for the mayor.

In addition to Kopp and Freeman, BU also awarded honorary degrees to Robert S. Langer, an MIT professor and pioneer of drug delivery systems, and Bishop Peter D. Weaver, former president of the Council of Bishops of the United Methodist Church and a former BU trustee.